Cron often runs in an almost empty environment.
The default path is very minimal. Therefore you should either use absolute paths or explicitly set the
PATH variable when using binaries.
PATH=/bin:/usr/bin:/usr/local/bin # or 5 4 * * sun /absolute/path/to/the/binary
If you decide to set the
PATH variable, you can’t append
PATH="$HOME/bin:$PATH" # does not work